Heat and humidity exchangers (also sometimes referred to as humidifiers) have been developed for a variety of applications, including building ventilation (HVAC), medical and respiratory applications, gas drying, and more recently for the humidification of fuel cell reactants for electrical power generation. Many such devices involve the use of a water-permeable membrane across which heat and moisture may be transferred between fluid streams flowing on opposite sides of the membrane.
Planar plate-type heat and humidity exchangers use membrane plates that are constructed of planar, water-permeable membranes (for example, Nafion®, cellulose, polymers or other synthetic membranes) supported with a spacer and/or frame. The plates are typically stacked, sealed and configured to accommodate intake and exhaust streams flowing in either cross-flow or counter-flow configurations between alternate plate pairs, so that heat and humidity are transferred between the streams via the membrane.
Other types of exchangers include hollow tube humidifiers and enthalpy wheel humidifiers. Hollow tube humidifiers have the disadvantage of high pressure drop, and enthalpy wheels tend to be unreliable because they have moving parts and tend to have a higher leak rate.
A heat recovery ventilator (HRV) is a mechanical device that incorporates a heat and humidity exchanger in a ventilation system for providing controlled ventilation into a building. The heat and humidity exchanger heats or cools incoming fresh air using exhaust air. Devices that also exchange moisture between the incoming fresh air and the exhaust air are generally referred to as Energy Recovery Ventilators (ERVs), sometimes also referred to as Enthalpy Recovery Ventilators. An ERV may remove excess humidity from the ventilating air that is being brought into a building or it may add humidity to the ventilating air. ERVs may be used to save energy and/or to improve indoor air quality in buildings.
ERVs typically comprise an enclosure, fans to move the air streams, ducting, as well as filters, control electronics and other components. The key component in the ERV which transfers the heat and humidity between the air streams is called the core or the exchanger. The two most common types of ERVs are those based on planar membrane plate-type devices and those based on rotating enthalpy wheel devices, both mentioned above. Planar plate-type ERV cores use layers of static plates that are sealed and configured to accommodate the intake and exhaust streams flowing in either cross-flow or counter-flow configurations between alternate pairs of plates.
FIG. 1 shows an example of a planar plate-type heat and humidity exchanger made from stacked planar sheets of membrane 3 with rigid corrugated spacers 6 inserted between the membrane sheets. The spacers maintain proper sheet spacing as well as defining airflow channels 5 for wet and dry streams on opposite sides of each membrane sheet, in a cross-flow arrangement, as indicated by broad arrows 1 and 2 respectively. The stack is encased within a rigid frame 4.
A benefit of planar plate-type heat and humidity exchanger designs for ERV, fuel cell, and other applications, is that they are readily scalable because the quantity (as well as the dimensions) of the modular membrane plates can be adjusted for different end-use applications. Existing planar plate-type ERV cores are bulky and less effective than would be desired in facilitating enthalpy exchange.
Another approach to heat and humidity exchanger design is to incorporate a pleated water-permeable material in the exchanger. For example, U.S. Pat. No. 4,040,804 describes a heat and moisture exchanger for exchanging heat and moisture between incoming and outgoing air for room ventilation. The exchanger has a cartridge containing a single pleated sheet of water-permeable paper. Air is directed in one direction along the pleats on one side of the pleated paper, and the return air flows in the opposite direction along the pleats on the other side of the pleated paper. The ends of the cartridge are closed by dipping them in wax or a potting compound that can be cast and that adheres to the paper. The pleats are separated or spaced, and air passages between the folds are provided, by adhering grains of sand to the pleated paper.
FIG. 2 shows an example of a heat and humidity exchanger suitable for energy recovery ventilator (ERV) applications which comprises a pleated water-permeable membrane cartridge disposed in a housing. A plastic flow field element can be disposed within some or all of the folds of the pleated membrane for directing the stream over the inner surfaces of the folds, as described in US Patent Application Publication No. 2008/0085437. The flow field element controls the relative flow paths of the two streams on opposite sides of the membrane and enhances flow distribution across one or both membrane surfaces. The flow field elements can also assist in supporting the pleated membrane and controlling the pleat spacing within the pleated membrane cartridge. In the embodiment shown in FIG. 2, a first fluid stream is directed in a U-shaped flow path 122 from an inlet port 124 on one face of housing 115 to an outlet port 128 on the same face of housing 115. The first fluid stream is thus directed from inlet port 124 into a set of substantially parallel folds 126 on one side of pleated membrane cartridge 120, then along the length of the folds 126, and then out via port 128. A second fluid stream is similarly directed in a substantially U-shaped flow path 132 from an inlet port 134 to an outlet port 138 on the same face of housing 115 (both ports 134 and 138 being on the opposite face of housing 115 from ports 124 and 128). The second fluid stream is directed from port 134 into a corresponding set of substantially parallel folds 136 on the other side of pleated membrane cartridge 120, then along the length of the folds 136, and then out via port 138. The flow path 122 of the first fluid stream is in a substantially counter-flow configuration relative to flow path 132 of the second fluid stream.
There are also examples of ERV cores with stacked planar membrane sheets that operate in a substantially counter-flow configuration to transfer heat and humidity across planar membrane sheets. The membrane sheets can be interleaved with rigid plastic spacers that define flow channels as described in U.S. Pat. No. 7,331,376.
The flow field inserts or spacers used in the heat and humidity exchangers described above often provide controlled or directional gas flow distribution over the membrane surface. However, the fluid flow paths across the membrane surface tend to be quite tortuous and turbulent, so the flow can be quite restricted and the pressure drop across the overall apparatus can be significant. If there are many closely-spaced ribs to support the membrane, the ribs will tend to impede or block the fluid flow, and also increase pressure drop. With more widely-spaced ribs the membrane can deflect into the channel also increasing the pressure drop. Therefore, the use of non-permeable flow field inserts is generally undesirable.
Compact heat and humidity exchangers or HRV cores in which there is heat transfer between channels in two dimensions in counter-flow are described in U.S. Pat. No. 5,725,051 in which the heat transfer medium is a thermoformed rigid plastic sheet. The plastic is impermeable to water so there is no humidity transfer across the medium. In another similar example, the heat transfer medium is aluminum, but again there is no humidity transfer because the medium is not water-permeable.
As described above, conventional ERV cores with a water-permeable membrane require a spacer to support the membrane. Spacers generally impede or block heat and moisture transfer and they can increase the pressure drop if there is deflection of the membrane into the channel.
The inventors have recognized that there remains a need for cost effective and efficient ERV systems and cores.